1. Field of the Invention
The present invention generally relates to a transmitter and a transmitting method in a spread spectrum mobile communications system, and in particular, to a spread spectrum signal generating device and method for maintaining a minimal transmitted output peak-to-average power.
2. Description of the Related Art
With the advent of CDMA (Code Division Multiple Access) mobile communications systems, various DSS (Direct Spread Spectrum) transmission and reception schemes have been explored. Coherent demodulation is known to be an effective way of increasing the subscriber capacity of a DSS-CDMA mobile communications system. This is largely due to a small signal-to-noise ratio which is generally required to obtain a given frame error rate in a coherent system, as compared to an incoherent demodulation system.
To realize coherent demodulation in a mobile communications environment, the complex gain of a received multipath channel signal on each path should be obtained. Complex gains can be calculated using a decision directed method or a pilot assisted method. The latter is generally used as it exhibits excellent performance and easy realization. The article entitled xe2x80x9cPerformance of Adaptive Match Filter Receivers Over Fading Multipath Channelsxe2x80x9d by Pahlavan and Matthews, IEEE Transactions on Communications, Vol. 38, No. 12, December 1990, pp. 2106-2113 provides more detailed information regarding the pilot assisted gain calculation method.
The pilot assisted method is implemented either by a parallel probing technique or serial probing technique. In parallel probing, a transmitter spreads a spread user data signal which includes both information and data known to a receiver with different PN (Pseudo random Noise) sequences. On the other hand, in serial probing, data known to the receiver is periodically inserted in the spread user data signal which includes information and then these signals are spread with the same PN symbol.
For CDMA mobile radio communications, a user needs to transmit various forms of data such as voice data, control data, and packet data for high-speed data and multimedia service. Two conditions need to be considered for such data transmission systems. First, it is desirable to minimize a peak-to-average power ratio (PAR) at an output port of a communications terminal in order to decrease both power dissipation and manufacturing cost of the terminal. Second, intermittent output power from the terminal should be minimized as this may cause another device carried by a user, such as a hearing aid or a cardiac pacemaker, to malfunction. The serial probing method is inferior to the parallel probing in terms of moderating intermittent output power, but offers advantages over the parallel probing method in terms of PAR.
FIG. 1 is a block diagram of a transmitter for generating a transmission signal including a pilot signal on a reverse link in a point-to-point spread spectrum CDMA cellular communications system.
Referring to FIG. 1, a logical channel data generator 111 has a plurality of data generators for generating channel data, and a plurality of scramblers for scrambling the channel data. A channelizer 113 processes the data received from the logical channel data generator 111 in such a manner that both interference between channels and the PAR is minimized. An IQ signal mapper 115 maps the channelized signals received from the channelizer 113 into in-phase and quadrature-phase signals. A PN spreader 117 spreads the output of the IQ signal mapper 115 with PN codes. A baseband modulator 119 translates the spread signal received from the PN spreader 117 to a baseband signal and modulates the baseband signal. A frequency upconverter 121 upconverts the frequency of the modulated signal received from the baseband modulator 119 to a transmission frequency and outputs a radio transmission signal.
FIG. 2A is a block diagram of the logical channel data generator 111 shown in FIG. 1, and FIG. 2B is a block diagram of the scramblers shown in FIG. 2A.
Referring to FIG. 2A, the logical channel data generator 111 includes a pilot data generator 211, a control data generator 213, a voice data generator 215, and a packet data generator 217. The pilot data generator 211 outputs unmodulated consecutive bits 0s. Control data generated from the control data generator 213 is composed of a power control command for power control on a forward link or other control information. The voice data generator 215 outputs data from a variable bit rate (VBR) vocoder. The voice data output from the vocoder can be, for example, a convolutionally encoded and interleaved bit sequence. The encoded voice data is output at a VBR of 1/2, 1/4, or 1/8, increasing the period of one bit time by two times, four times, or eight times, respectively. The packet data generator 217 has an output bit rate which is an integer multiple (from 1 to 8) of the highest bit rate of the voice data generator 215.
Scramblers 219, 221, and 223 scramble the data received from the control data generator 213, the voice data generator 215, and the packet data generator 217, respectively.
Referring to FIG. 2B, a switch 232 of the scramblers 219, 221, or 223 selectively outputs the output of a decimator 233 or data xe2x80x9c0xe2x80x9d, and an exclusive OR gate 231 exclusive-ORs the data received from the data generators 213, 215, or 217 with the output of the decimator 233 or the data xe2x80x9c0xe2x80x9d selected by the switch 232. The decimator 233 decimates a second PN code sequence (i.e., long PN code sequence) P at the same bit rate as that of the control, voice, and packet data, which were all encoded and interleaved.
FIGS. 3A and 3B are block diagrams of the channelizer 113 shown in FIG. 1, which are configured for the serial and parallel probe methods, respectively. Referring to FIG. 3A, rate adaptors 311 to 317 are connected to the respective data generators 211 to 217 and adjust the data rates at the data generators 211 to 217. Signal mappers 321 to 327, which are connected to the respective rate adaptors 311 to 317, convert bits 0s and 1s of rate-adapted data to +1s and xe2x88x921s, respectively. Multipliers 331 to 337 multiply the converted signals received from the signal mappers 321 to 327 by corresponding channel amplitude control signals A0 to A3. A multiplexer 341 multiplexes the outputs of the multipliers 331 to 337.
In the channelizer 113 using the serial probe scheme, the various data is time multiplexed to an output C0 to occupy a different time slot therein and the time that the data occupies is adjusted by varying the number of repetitions on the outputs of the data generators 211 to 217 in the rate-adaptors 311 to 317.
The rate-adapted data is converted from logical channel data 0s and 1s to +1s and xe2x88x921s suitable for transmission by the signal converters 321 to 327. The output signal from the signal converters 321 to 327 are applied to the multipliers 331 to 337, which multiply the converted signals by channel amplitude control signals A0 to A3, thereby determining the power levels.
Referring to FIG. 3B, rate adaptors 351 to 357 are connected to the data generators 211 to 217 of the logical channel data generator 111 and adjust data transmission rates at the corresponding data generators 211 to 217. Signal mappers 361 to 367 are connected to the corresponding rate adaptors 351 to 357, for converting bits 0s and 1s of rate-adapted data to +1s and xe2x88x921s, respectively. Walsh code generators 371 to 377 generate Walsh codes W0 to W3, respectively. Multipliers 381 to 387 multiply signals received from the signal mappers 321 to 327 by the Walsh codes W0 to W3 received from the Walsh code generators 371 to 377, to remove both interference between channels and phase errors. Multipliers 391 to 397 multiply the outputs of the multipliers 381 to 387 by the corresponding channel amplitude control signals A0 to A3, thereby controlling the channel amplitude of the signals.
In the channelizer 113 using the parallel probe method, the occupation time of each data is adjusted by varying the number of repetitions on the outputs of the data generators 211 to 217 by the rate adaptors 351 to 357. The rate-adapted data is converted from logical channel data 0s and 1s to +1s and xe2x88x921s suitable for transmission by the signal mappers 361 to 367, and multiplied by the mutually orthogonal Walsh codes by the multipliers 381 to 387, thereby reducing interference between channels and phase error-induced performance deterioration. The outputs of the multipliers 381 to 387 are multiplied by the corresponding channel amplitude control signals A0 to A3 by the multipliers 391 to 397 so that power levels are determined.
FIG. 4A is a block diagram of the IQ signal mapper 115 shown in FIG. 1, which is connected to the channelizer 113 when implementing the serial probe scheme. FIG. 4B is a block diagram of the IQ signal mapper 115 shown in FIG. 1, which is connected to the channelizer 113 for the parallel probe scheme. The IQ signal mapper 115 maps a channelized signal into both in-phase and quadrature-phase signals.
Because the final output C0 of the channelizer 113 using the serial probe scheme is multiplexed data, the IQ signal mapper 115 of FIG. 4A is provided with a serial-to-parallel converter 411 for separating the multiplexed signal into odd-numbered bits and even-numbered bits and generating an in-phase signal (I signal) and a quadrature-phase signal (Q signal).
Since the final output of the channelizer 113 using the parallel probe scheme is non-multiplexed parallel data, the IQ signal mapper 115 of FIG. 4B includes a first adder 421 for adding the pilot channel signal C0 and the voice channel signal C2 and thus generating an I signal, and a second adder 423 for adding the control channel signal C1 and the packet channel signal C3 and thus generating a Q signal.
FIG. 5A is a block diagram of the PN spreader 117 shown in FIG. 1 using an IQ split method, and FIG. 5B is a block diagram of the PN spreader 117 shown in FIG. 1 using a complex spreading method. Here, a first PN code refers to a short PN code, and a second PN code refers to a long PN code.
Referring to FIG. 5A, a first PNi code generator 511 generates an in-phase PN code PNi and a first PNq code generator 513 generates a quadrature-phase PN code PNq. A second PN code generator 515 generates a long PN code commonly applied to the in-phase PN code, PNi and the quadrature-phase PN code, PNq. A multiplier 517 multiplies PNi by the second PN code, thereby generating an in-phase PN code. A multiplier 519 multiplies PNq by the second PN code, thereby generating a quadrature-phase PN code. A multiplier 520 multiplies the I signal received from the IQ signal mapper 115 by the quadrature-phase PN code and generates a spread signal PI. A multiplier 512 multiplies the Q signal received from the IQ signal mapper 115 by the in-phase PN code and generates a spread signal PQ.
Now, there will be a description of the PN spreader 117 for complex spreading shown in FIG. 5B. Referring to FIG. 5B, the first PNi code generator 511 generates the in-phase PN code PNi and the first PNq code generator 513 generates a quadrature-phase PN code PNq. The second PN code generator 515 generates a long PN code which is applied to both the PNi and PNq PN codes. The multiplier 517 multiplies PNi by the second PN code, thereby generating an in-phase PN code. Similarly, multiplier 519 multiplies PNq by the second PN code, thereby generating a quadrature-phase PN code. A multiplier 521 multiplies the I signal received from the IQ signal mapper 115 by the in-phase PN code. A multiplier 523 multiplies the Q signal received from the IQ signal mapper 115 by the in-phase PN code. A multiplier 525 multiplies the Q signal received from the IQ signal mapper 115 by the quadrature-phase PN code. A multiplier 527 multiplies the I signal received from the IQ signal mapper 115 by the quadrature-phase PN code. A subtracter 529 subtracts the output of the multiplier 525 from the output of the multiplier 521 and generates a complex-spread in-phase signal PI. An adder 531 adds the outputs of the multipliers 523 and 527 and generates a complex-spread quadrature-phase signal PQ. The PN spreader 117 of FIG. 5B offers a lower peak-to-average power ratio as compared to the topology of FIG. 5A.
FIG. 6 further illustrates the baseband modulator 119 which modulates the spread signals PI and PQ received from the PN spreader 117 shown in FIG. 5A or 5B. Referring to FIG. 6, the spread signal PI is filtered by an FIR (Finite Impulse Response) filter 615, whereas the spread signal PQ is delayed by a predetermined time in a delay 611 and filtered by an FIR filter 613. The baseband modulator 119 may operate based on OQAM (Offset Quadrature Amplitude Modulation).
A transmitter using the parallel probe method includes the channelizer 113 of FIG. 3B, the IQ signal mapper 115 of FIG. 4B, the PN spreader 117 of FIG. 5B, and the baseband modulator 119 of FIG. 6. On the other hand, a transmitter employing the serial probe method has the channelizer 113 of FIG. 3A, the IQ signal mapper 115 of FIG. 4A, the PN spreader 117 of FIG. 5A, and the baseband modulator 119 of FIG. 6.
The transmitter using the parallel probe method increases PAR, while transmitters using the serial probe method suffers a significant power variation due to a varied bit rate of a voice signal and the intermittent presence of a packet signal, thereby increasing interference.
Therefore, concurrent use of multiple channels gives rise to problems associated with an amplifier in the conventional transmitters. That is, because the pilot channel, the control channel, the voice channel, and the packet channel are simultaneously used, a peak-to-average power ratio is increased, which implies that the amplifier should exhibit excellent linearity. In particular, a terminal using only the voice channel (i.e., low speed traffic channel) without the packet channel (i.e., high speed traffic channel) may have a seriously increased peak-to-average power ratio depending on gain adjustment for channels.
An object of the present invention is to provide a spread spectrum signal generating device and method in a mobile communications system for transmitting data of multiple logical channels, where the data of logical channels having constant transmit power levels is channelized by multiplexing and orthogonal codes.
Another object of the present invention is to provide a spread spectrum signal generating device in a mobile communications system for transmitting data of multiple logical channels, where the data of logical channels having constant transmit power levels is channelized by multiplexing and the data of the other logical channels is channelized on the basis of the power level of the multiplexed channel.
To achieve the above objects, a spread spectrum signal generating device in a transmitter of a mobile communications system is provided which uses a plurality of logical channels. In the spread spectrum signal generating device, a multiplexer time multiplexes a pilot channel signal and a control channel signal which are output at constant power levels. A first orthogonal encoder orthogonally spreads the output of the multiplexer with an orthogonal code, a second orthogonal encoder orthogonally spreads voice channel data of a variable bit rate with an orthogonal code, a third orthogonal encoder orthogonally spreads packet channel data of a variable bit rate with an orthogonal code, an IQ signal mapper adds the outputs of the first and third orthogonal encoders, outputs the added signal as a first channel signal, and outputs the output of the second orthogonal encoder as a second channel signal, and a PN spreader spreads the first and second channel signals with PN codes and outputs a final spectrum spread signal. The resulting spread spectrum signal features a substantially uniform peak-to-average power ratio.